(1) Field of the Invention
The present invention relates to compositions and methods for producing proteins having specific glycosylation patterns. In particular, the present invention relates to compositions and methods for producing proteins having reduced O-linked glycosylation.
(2) Description of Related Art
Glycoproteins mediate many essential functions in humans and other mammals, including catalysis, signaling, cell-cell communication, and molecular recognition and association. Glycoproteins make up the majority of non-cytosolic proteins in eukaryotic organisms (Lis and Sharon, 1993, Eur. J. Biochem. 218:1-27). Many glycoproteins have been exploited for therapeutic purposes, and during the last two decades, recombinant versions of naturally-occurring glycoproteins have been a major part of the biotechnology industry. Examples of recombinant glycosylated proteins used as therapeutics include erythropoietin (EPO), therapeutic monoclonal antibodies (mAbs), tissue plasminogen activator (tPA), interferon-β (IFN-β), granulocyte-macrophage colony stimulating factor (GM-CSF), and human chorionic gonadotrophin (hCH) (Cumming et al., 1991, Glycobiology 1:115-130). Variations in glycosylation patterns of recombinantly produced glycoproteins have recently been the topic of much attention in the scientific community as recombinant proteins produced as potential prophylactics and therapeutics approach the clinic.
In general, the glycosylation structures of glycoprotein oligosaccharides will vary depending upon the host species of the cells used to produce them. Therapeutic proteins produced in non-human host cells are likely to contain non-human glycosylation which may elicit an immunogenic response in humans—e.g. hypermannosylation in yeast (Ballou, 1990, Methods Enzymol. 185:440-470); α(1,3)-fucose and β(1,2)-xylose in plants, (Cabanes-Macheteau et al., 1999. Glycobiology, 9: 365-372); N-glycolylneuraminic acid in Chinese hamster ovary cells (Noguchi et al., 1995. J. Biochem. 117: 5-62); and, Galα-1,3Gal glycosylation in mice (Borrebaeck, et al., 1993, Immun Today, 14: 477-479). Carbohydrate chains bound to proteins in animal cells include N-glycoside bond type carbohydrate chains (also called N-glyeans; or N-linked glycosylation) bound to an asparagine (Asn) residue in the protein and O-glycoside bond type carbohydrate chains (also called O-glycans; or O-linked glycosylation) bound to a serine (Ser) or threonine (Thr) residue in the protein.
Because the oligosaccharide structures of glycoproteins produced by non-human mammalian cells tend to be more closely related to those of human glycoproteins, most commercial glycoproteins are produced in mammalian cells. However, mammalian cells have several important disadvantages as host cells for protein production. Besides being costly, processes for producing proteins in mammalian cells produce heterogeneous populations of glycoforms, have low volumetric titers, and require both ongoing viral containment and significant time to generate stable cell lines.
It is well recognized that the particular glycoforms on a protein can profoundly affect the properties of the protein, including its pharmacokinetic, pharmacodynamic, receptor-interaction, and tissue-specific targeting properties (Graddis et al., 2002. Curr Pharm Biotechnol. 3: 285-297). For example, it has been shown that different glycosylation patterns of Igs are associated with different biological properties (Jefferis and Lund, 1997, Antibody Eng. Chem. Immunol., 65: 111-128; Wright and Morrison, 1997, Trends Biotechnol., 15: 26-32). It has further been shown that galactosylation of a glycoprotein can vary with cell culture conditions, which may render some glycoprotein compositions immunogenic depending on the specific galactose pattern on the glycoprotein (Patel et al., 1992. Biochem J. 285: 839-845). However, because it is not known which specific glycoform(s) contribute(s) to a desired biological function, the ability to enrich for specific glycoforms on glycoproteins is highly desirable. Because different glycoforms are associated with different biological properties, the ability to enrich for glycoproteins having a specific glycoform can be used to elucidate the relationship between a specific glycoform and a specific biological function of the glycoprotein. Also, the ability to enrich for glycoproteins having a specific glycoform enables the production of therapeutic glycoproteins having particular specificities. Thus, production of glycoprotein compositions that are enriched for particular glycoforms is highly desirable.
While the pathway for N-linked glycosylation has been the subject of much analysis, the process and function of O-linked glycosylation is not as well understood. However, it is known that in contrast to N-linked glycosylation, O-glycosylation is a posttranslational event, which occurs in the cis-Golgi (Varki, 1993, Glycobiol., 3: 97-130). While a consensus acceptor sequence for O-linked glycosylation like that for N-linked glycosylation does not appear to exist, a comparison of amino acid sequences around a large number of O-linked glycosylation sites of several glycoproteins show an increased frequency of proline residues at positions −1 and +3 relative to the glycosylated residues and a marked increase of serine, threonine, and alanine residues (Wilson et al., 1991, Biochem. J., 275: 529-534). Stretches of serine and threonine residues in glycoproteins, may also be potential sites for O-glycosylation.
One gene family that has a role in O-linked glycosylation are the genes encoding the Dol-P-Man:Protein (Ser/Thr) Mannosyl Transferase (Pmt). These highly conserved genes have been identified in both higher eukaryotes such as humans, rodents, insects, and the like and lower eukaryotes such as fungi and the like. Yeast such as Saccharomyces cerevisiae and Pichia pastoris encode up to seven PMT genes encoding Pmt homologues (reviewed in Willer et al. Curr. Opin. Struct. Biol. 2003 October; 13(5): 621-30.). In yeast, O-linked glycosylation starts by the addition of the initial mannose from dolichol-phosphate mannose to a serine or threonine residue of a nascent glycoprotein in the endoplasmic reticulum by one of the seven O-mannosyl transferases genes. While there appear to be seven PMT genes encoding Pmt homologues in yeast, O-mannosylation of secreted fungal and heterologous proteins in yeast is primarily dependent on the genes encoding Pmt1 and Pmt2, which appear to function as a heterodimer. PMT1 and PMT2 and their protein products, Pmt1 and Pmt2, respectively, appear to be highly conserved among species.
Tanner et al. in U.S. Pat. No. 5,714,377 describes the PMT1 and PMT2 genes of Saccharomyces cerevisiae and a method for making recombinant proteins having reduced O-linked glycosylation that uses fungal cells in which one or more of PMT genes have been genetically modified so that recombinant proteins are produced, which have reduced O-linked glycosylation.
Ng et al. in U.S. Published Patent Application No. 20020068325 discloses inhibition of O-glycosylation through the use of antisense or cosuppression or through the engineering of yeast host strains that have loss of function mutations in genes associated with O-linked glycosylation, in particular, one or more of the PMT genes.
UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetyl galactosaminyl-transferases (GalNAc-transferases) are involved in mucin type O-linked glycosylation found in higher eukaryotes. These enzymes initiate O-glycosylation of specific serine and threonine amino acids in proteins by adding N-acetylgalactosamine to the hydroxy group of these amino acids to which mannose residues can then be added in a step-wise manner. Clausen et al. in U.S. Pat. No. 5,871,990 and U.S. Published Patent Application No. 20050026266 discloses a family of nucleic acids encoding UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetyl galactosaminyl-transferases (GalNAc-transferases). Clausen in U.S. Published Patent Application No. 20030186850 discloses the use of GalNAc-beta-benzyl to selectively inhibit lectins of polypeptide GalNAc-transferases and not serve as substrates for other glycosyltransferases involved in O-glycan biosyntheses, thus inhibiting O-glycosylation.
Inhibitors of O-linked glycosylation have been described. For example, Orchard et al. in U.S. Pat. No. 7,105,554 describes benzylidene thiazolidinediones and their use as antimycotic agents, e.g., antifungal agents. These benzylidene thiazolidinediones are reported to inhibit the Pmt1 enzyme, preventing the formation of the O-linked mannoproteins and compromising the integrity of the fungal cell wall. The end result is cell swelling and ultimately death through rupture.
Konrad et al. in U.S. Published Patent Application No. 20020128235 disclose a method for treating or preventing diabetes mellitus by pharmacologically inhibiting O-linked protein glycosylation in a tissue or cell. The method relys on treating a diabetic individual with (Z)-1-[N-(3-Ammoniopropyl)-N-(n-propyl)amino]diazen-ium-1,2-diolate or a derivative thereof, which binds O-linked N-acetylglucosamine transferase and thereby inhibits O-linked glycosylation.
Kojima et al. in U.S. Pat. No. 5,268,364 disclose therapeutic compositions for inhibition of O-glycosylation using compounds such as benzyle-α-N-acetylgalactosamine, which inhibits extension of O-glycosylation leading to accumulation of O-α-GalNAc, to block expression of SLex or SLea by leukocytes or tumor cells and thereby inhibit adhesion of these cells to endothelial cells and platelets.
Boime et al. U.S. Pat. No. 6,103,501 disclose variants of hormones in which O-linked glycosylation was altered by modifying the amino acid sequence at the site of glycosylation.
The present inventors have found that particular chemical compounds that are inhibitors of Pmt proteins, which are generally lethal to fungi, can be used in a way which is not lethal to the host cells for production of recombinant proteins with reduced O-linked glycosylation. This enables O-linked glycosylation of proteins produced from fungi and yeast cells to be controlled. Other classes of chemical compounds, which the inventors believe to be non-lethal inhibitors of the PMT enzymes, are also useful in the production of improved glycoproteins with reduced O-linked glycosylation. The present inventors have further found that addition to the host cell or cell culture of certain classes of enzymes, namely, α-1,2-mannosidases, alone or in combination with a chemical inhibitor of Pmt proteins effects a further reduction of O-glycosylation.